US 20060078040 A1 Abstract An apparatus and method for cell acquisition and downlink synchronization acquisition in an OFDMA wireless communication system are provided. In an SS apparatus in a broadband wireless communication system, a preamble subcarrier acquirer extracts subcarrier values having a preamble code from an FFT signal. A multiplier code-demodulates the subcarrier values by multiplying the subcarrier values by a preamble code. A correlator calculates a plurality of differential correlations in the code-demodulated signal. An IFFT processor IFFT-processes the differential correlations by mapping the differential correlations to subcarriers. A maximum value detector detects a maximum value from the IFFT signal and calculates a timing offset using an IFFT output index having the maximum value.
Claims(23) 1. A subscriber station (SS) apparatus in a broadband wireless communication system, comprising:
a preamble subcarrier acquirer for extracting subcarrier values having a preamble code from a fast Fourier transform (FFT) signal; a multiplier for code-demodulating the subcarrier values by multiplying the subcarrier values by a preamble code; a correlator for calculating a plurality of differential correlations in the code-demodulated signal; an inverse fast Fourier transform (IFFT) processor for IFFT-processing the differential correlations by mapping the differential correlations to subcarriers and outputting an IFFT signal; and a maximum value detector for detecting a maximum value from the IFFT signal and calculating a timing offset using an IFFT output index having the maximum value. 2. The SS apparatus of _{max }differential correlations by performing an n^{th}-order differential correlation on the code-modulated signal (1≦n≦n_{max}). 3. The SS apparatus of a peak-to-average power ratio (PAPR) calculator for calculating the PAPR of the IFFT signal; and a comparator for comparing the PAPR with a threshold and setting the calculated timing offset as a timing offset estimate if the PAPR is greater than the threshold. 4. The SS apparatus of 5. The SS apparatus of 6. The SS apparatus of a first-order differential correlator for calculating a differential correlation between two adjacent subcarriers over all the cases in each of the code-demodulated signals, summing the differential correlations as a correlation for each of the code-demodulated signals, and outputting a plurality of correlations for the code-demodulated signals; and a detector for detecting the highest of the correlations and acquiring a cell having the highest correlation. 7. The SS apparatus of 8. The SS apparatus of a first-order differential correlator for calculating a differential correlation between two adjacent subcarriers over all the cases in each of the code-demodulated signals, summing the differential correlations as a correlation for each of the code-demodulated signals, and outputting a plurality of correlations for the code-demodulated signals; and an arranger for prioritizing the neighbor cells as handoff target cells by arranging the correlations in a predetermined order. 9. The SS apparatus of _{j }is the index of a subcarrier to which a j^{th }preamble code bit is mapped, Y_{IDcell,s}(K_{j}) is the received signal response of a K_{j} ^{th }subcarrier, and W is a window size for the differential correlator, the correlator calculates the 2×n_{max }correlations (Z_{n}) by 10. The SS apparatus of _{max }is determined according to the coherence bandwidth of a channel and is less than a half of an IFFT size of the IFFT processor. 11. The SS apparatus of _{FFT}/J, and d is the spacing between two adjacent subcarriers, the maximum value detector calculates the timing offset (Δt_{offset}) by 12. The SS apparatus of 13. A receiving method for a subscriber station (SS) in a broadband wireless communication system, comprising the steps of:
acquiring subcarrier values having a preamble code from a fast Fourier transform (FFT) signal; code-demodulating the subcarrier values by multiplying the subcarrier values by a preamble code; calculating a plurality of differential correlations in the code-demodulated signal; inverse fast Fourier transform (IFFT)-processing the differential correlations by mapping the differential correlations to subcarriers and outputting an IFFT signal; and detecting a maximum value from the IFFT signal and calculating a timing offset using an IFFT output index having the maximum value. 14. The receiving method of ^{th}-order differential correlations by calculating a differential correlation between subcarriers spaced by n (≧1) over all the cases in the code-demodulated signal and summing the calculated differential correlations. 15. The receiving method of calculating the peak-to-average power ratio (PAPR) of the IFFT signal; and verifying the calculated timing offset by comparing the PAPR with a threshold. 16. The receiving method of 17. The receiving method of _{j }is the index of a subcarrier to which a j^{th }preamble code bit is mapped, Y_{IDcell,s}(K_{j}) is the received signal response of a K_{j} ^{th }subcarrier, and W is a window size for the differential correlator, the differential correlation step comprises the step of calculating the differential correlations (Z_{n}) by 18. The receiving method of _{max }is determined according to the coherence bandwidth of a channel and is less than a half of an IFFT size. 19. The receiving method of _{FFT}/J, and d is the spacing between two adjacent subcarriers, the timing offset calculation step comprises the step of calculating the timing offset (Δt_{offset}) by 20. An initial cell search method in a broadband wireless communication system, comprising the steps of:
performing a first-order differential correlation on each of signals demodulated with all possible preamble codes and acquiring a cell having the highest first-order correlation, at an initial cell search; calculating a plurality of differential correlations by performing an n ^{th}-order differential correlation on the code-modulated signal of the acquired cell (1≦n≦n_{max}); inverse fast Fourier transform (IFFT)-processing the differential correlations by mapping the differential correlations to subcarriers and outputting an IFFT signal; and detecting a maximum value from the IFFT signal and calculating a timing offset using an IFFT output index having the maximum value. 21. The initial cell search method of calculating the peak-to-average power ratio (PAPR) of the IFFT signal; and verifying the calculated timing offset by comparing the PAPR with a threshold. 22. A handoff target cell search method in a broadband wireless communication system, comprising the steps of:
performing a first-order differential correlation on each of signals demodulated with the preamble codes of known neighbor cells and ordering the neighbor cells according to the first-order differential correlations, at a handoff target cell search; calculating a plurality of differential correlations by performing an n ^{th}-order differential correlation on each of the code-modulated signals of the neighbor cells (1≦n≦n_{max}); inverse fast Fourier transform (IFFT)-processing the differential correlations for each neighbor cell by mapping the differential correlations to subcarriers and outputting an IFFT signal for the each neighbor cell; and detecting a maximum value from the IFFT signal of the each neighbor cell and calculating a timing offset for the each neighbor cell using an IFFT output index having the maximum value. 23. The handoff target cell search method of Description This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus And Method For Cell Acquisition And Downlink Synchronization Acquisition In A Wireless Communication System” filed in the Korean Intellectual Property Office on Oct. 12, 2004 and assigned Serial No. 2004-81313, the contents of which are incorporated herein by reference. 1. Field of the Invention The present invention relates generally to a receiving apparatus and method for a Subscriber Station (SS) in an Orthogonal Frequency Division Multiplexing (OFDM)-based broadband wireless communication system, and in particular, to an apparatus and method for cell acquisition and downlink synchronization acquisition in a Time Division Duplex-Orthogonal Frequency Division Multiple Access (TDD-OFDMA) communication system. 2. Description of the Related Art In OFDMA communication systems based on the IEEE 802.16d/e standard, an SS identifies a cell and acquires synchronization to the cell using a pilot signal or a preamble signal received from a Base Station (BS). Any signal transmitted from the BS for assisting in cell acquisition and downlink synchronization in the SS is referred to as a “preamble signal”. Cell acquisition and synchronization acquisition in IEEE 802.16d/e based systems will be described. Referring to In the OFDMA communication system having the above-described configuration, upon power-on, an SS attempts to access a BS. The SS first acquires synchronization to receive information from the BS. The synchronization is the process of detecting the absolute time of a received signal and acquiring a signal in a desired time period among successively received signals. The synchronization is divided into primary rough synchronization referred to as frame synchronization, and secondary fine synchronization for acquiring accurate timing synchronization. For a wireless connection, the SS performs a cell acquisition in which it estimates information about a BS to transmit the most acceptable signal to the SS, after frame synchronization. Then the SS carries out fine synchronization with the BS to prevent degradation of a received signal from arising from asynchronization. Once the SS is connected to the BS, the SS transmits/receives data to/from the BS. If the SS roams (i.e. the wave propagation time varies according to the moved distance) or the clock signal from the SS does not trigger and instead drifts, the SS loses the initially acquired downlink timing synchronization. To solve this out-of-synchronization problem, the SS prevents a received signal from being degraded through periodic synchronization (synchronization tracking) while connected to the BS. Also, if the SS moves away from the BS after the connection, the Signal-to-Noise Ratio (SNR) of a signal received from the BS decreases. That is, as the SS is farther from the BS, path loss increases, causing handoff. The SS then attempts a handoff and tries to access a BS that offers the highest SNR. During this operation, the SS performs synchronization and neighbor cell acquisition. If the cell acquisition and synchronization take place after the handoff is decided, a long time delay occurs until the handoff is completed. The SS must acquire information about neighbor cells (SNR, timing offsets, and cell acquisition) beforehand in preparation for the handoff. Now a description will be made of how the preamble signal is transmitted. Referring to
An apparatus for transmitting a preamble with the above characteristics will be described. Referring to Initial cell search using the preamble signal will be described below. Referring to The above-described cell search is modeled as Equation (1):
After the cell acquisition, the timing offset of the cell is detected. The timing offset detection is performed by Equation (2):
Referring to Equation (2), Y Acquisition of handoff information using the preamble signal will be described now. Referring to The above-described neighbor cell search is modeled as Equation (3):
The timing offset of the acquired neighbor cells are detected according to their priority levels by Equation (4):
The parameters in Equation (4) and Equation (2) are alike in their definitions. How synchronization to a home cell is tracked using the preamble signal will be described. Referring to The above-described synchronization tracking for the home cell is modeled as Equation (5):
The parameters shown in Equation (5) and Equation (2) are alike in their definitions. As described above, the conventional TDD-OFDMA communication system searches cells and acquires timing offsets as illustrated in (1) Due to too much computation volume, real implementation is difficult. Table 2 below lists computation volumes for cell acquisition and timing offset acquisition in the conventional technology, especially based on the Korean 2.3 GHz WiBro physical layer standard.
(2) The reception performance of a preamble decreases considerably at low SNR. In Equation (2), the subcarrier reception response Y An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for reducing a computation volume required for cell search in an OFDM wireless communication system. Another object of the present invention is to provide an apparatus and method for reducing a computation volume required for downlink synchronization acquisition in an OFDM wireless communication system. A further object of the present invention is to provide an apparatus and method for reducing a computation volume required for neighbor cell search for handoff in an OFDM wireless communication system. Still another object of the present invention is to provide an apparatus and method for performing all of the initial cell search, neighbor cell search, and downlink synchronization acquisition by a single hardware configuration in an OFDM wireless communication system. Yet another object of the present invention is to provide an apparatus and method for increasing preamble detection performance in an OFDM wireless communication system. Yet further object of the present invention is to provide an apparatus and method for increasing downlink synchronization acquisition performance in an OFDM wireless communication system. The above objects are achieved by providing an apparatus and method for cell acquisition and downlink synchronization acquisition in an OFDMA wireless communication system. According to one aspect of the present invention, in an SS apparatus in a broadband wireless communication system, a preamble subcarrier acquirer extracts subcarrier values having a preamble code from an FFT signal. A multiplier code-demodulates the subcarrier values by multiplying the subcarrier values by a predetermined preamble code. A correlator calculates a plurality of differential correlations in the code-demodulated signal. An IFFT processor IFFT-processes the differential correlations by mapping the differential correlations to predetermined subcarriers. A maximum value detector detects a maximum value from the IFFT signal and calculates a timing offset using an IFFT output index having the maximum value. According to another aspect of the present invention, in a receiving method for an SS in a broadband wireless communication system, subcarrier values having a preamble code are acquired from an FFT signal and code-demodulated by multiplying the subcarrier values by a preamble code. A plurality of differential correlations are calculated from the code-demodulated signal and IFFT-processed by mapping the differential correlations to subcarriers. A maximum value is detected from the IFFT signal and a timing offset is calculated using an IFFT output index having the maximum value. According to a further aspect of the present invention, in an initial cell search method in a broadband wireless communication system, a first-order differential correlation is performed on signals demodulated with all possible preamble codes and a cell having the highest first-order correlation is acquired. A plurality of differential correlations are calculated by performing an n According to still another aspect of the present invention, in a handoff target cell search method in a broadband wireless communication system, a first-order differential correlation is performed on each of signals demodulated with the preamble codes of known neighbor cells and the neighbor cells are ordered according to the first-order differential correlations. A plurality of differential correlations are calculated by performing an n The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. The present invention is intended to provide a method of reducing computation volume required for cell search and synchronization tracking, and improving preamble detection performance at low SNR in an OFDMA communication system. In the OFDMA communication system, an SS identifies a cell and acquires downlink synchronization using a signal such as a pilot or a preamble. The present invention as described below is applicable to any OFDMA system that identifies a cell and acquires downlink synchronization using a signal such as a pilot or a preamble. A description will first be made of the configuration of downlink preamble subcarriers. Referring to Initial cell search using the preamble signal will be described below. The initial cell search apparatus of the present invention includes an FFT processor Referring to The first-order differential correlator The maximum value detector The N The zero padder The J-point IFFT processor As illustrated in The IFFT signal Z Therefore, the maximum value detector The operations of the J-point IFFT processor To verify the timing offset from the maximum value detector The PAPR comparator Referring to In step The SS calculates n In step Since the resulting IFFT signal is a sinc function, the SS detects a maximum value from the IFFT signal and calculates a timing offset using an IFFT output index having the maximum value in step The SS calculates the PAPR of the IFFT signal in step Acquisition of handoff information using the preamble signal will be addressed now. Referring to Referring to Each of the multipliers Each of the first-order differential correlators The descending-order arranger For each of the preamble code-demodulated signals of the arranged cells, the N The zero padder The J-point IFFT processor As illustrated in Therefore, the maximum value detector The operations of the N Referring to The SS sets a variable n indicating the index of a neighbor cell in step In step Thus, the SS detects a maximum value from the IFFT signal and calculates a timing offset using an IFFT output index having the maximum value in step The SS compares the variable n with N in step Tracking the synchronization of the home cell will be described now. The synchronization tracking apparatus of the present invention includes an FFT processor Referring to The N The zero padder The J-point IFFT processor As illustrated in Therefore, the maximum value detector To verify the timing offset from the maximum value detector Referring to In step Therefore, the SS detects a maximum value from the IFFT signal and calculates a timing offset using an IFFT output index having the maximum value in step In step As described above, the initial cell search apparatus illustrated in Compared to the conventional technology, the cell search and synchronization tracking according to the present invention require a far less computation volume. A comparison in computation volume or complex between the present invention and the conventional technology is given in Table 3 below.
As noted from Table 3, an initial cell search requires almost the same computation volume in both the conventional technology and the present invention. The computation volume required for a neighbor cell search for handoff in the present invention is 77.76% of that in the conventional technology, an about 22% decrease. Furthermore, as to the computation requirement for synchronization tracking, the present invention is no more than 32.01% of the conventional technology. In accordance with the present invention as described above, implementation complexity is remarkably reduced because cell search and synchronization acquisition can be implemented by use of a single apparatus. As noted from Table 3, the computation requirement is also significantly decreased compared to the conventional technology. The synchronization acquisition is performed such that a correlation decrease caused by the timing offset between subcarriers on the frequency axis is eliminated. Therefore, timing synchronization is correctly detected. In addition, the detected timing synchronization is verified using a PAPR, leading to more reliable timing synchronization. While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Referenced by
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